1. Field of the Invention
The present invention relates to an epitaxial substrate prepared by epitaxially growing a group III nitride semiconductor on an SiC base, and to an HEMT device using the same.
2. Description of the Background Art
Nitride semiconductors having higher breakdown electric field and higher electron saturation velocities have been drawing attention as semiconductor materials for high-frequency and high-power devices. In manufacturing such devices, a group III nitride semiconductor is generally epitaxially grown on a predetermined base substrate. While an SiC substrate, a sapphire substrate or the like is mainly used as a base substrate, defect density of the group III nitride semiconductor grown thereon can be much more reduced (crystal quality is further improved) when using the SiC substrate than when using the sapphire substrate since SiC has a lattice constant closer to AlN and GaN than sapphire. Further, an SiC substrate has excellent heat conductivity. Thus, high output power and high-frequency characteristics of devices are expected to be improved by using the SiC substrate.
On the other hand, a technique to use an AlN film as a buffer layer in forming a GaN film on the SiC substrate has been already well known (for example, see National Publication of Translation No. 2002-520880, Japanese Patent Application Laid-Open No. 2006-286741, and Japanese Patent Application Laid-Open No. 2004-200384).
As disclosed in National Publication of Translation No. 2002-520880, Japanese Patent Application Laid-Open No. 2006-286741, and Japanese Patent Application Laid-Open No. 2004-200384, a technique of forming an AlN film on the SiC substrate as a buffer layer, and forming a GaN film thereon is well known. However, National Publication of Translation No. 2002-520880 and Japanese Patent Application Laid-Open No. 2006-286741 do not disclose any of the relationship of a specific formation of an AlN film functioning as a buffer layer to crystal quality of a GaN film, and further to a characteristics of an electronic device configured to form a functional layer with the GaN film or the like.
The inventors of the present invention have made it clear that cracks are easy to be generated in a channel layer when a buffer layer is formed of AlN having excellent crystal quality, as disclosed in Japanese Patent Application Laid-Open No. 2004-200384, and the channel layer formed of GaN is grown.